This invention relates to electrical switching circuits and more particularly to solid state switching circuits for controlling DC power in electrical systems.
Solid state switches are of interest for use in aircraft power systems as well as other applications to secure the recognized advantages of solid state components over electromechanical circuit breaker devices. One basic function of such switches is to serve as a remote power controller; that is, to permit switching of the power to a load from a remote location. DC remote power controllers use controlled current limiting to eliminate transient inrush currents, thereby protecting the load and associated wiring from high fault currents. If a fault, and consequently the current limiting condition, persists for a predetermined length of time, a trip circuit within the remote power controller will trip and latch off the associated solid state switching device. Then the load and fault is disconnected from the power system bus and will remain disconnected until the remote power controller is reset.
During the current limiting period, the static switch within the remote power controller must dissipate a power level equal to the switch voltage drop times the load current. The maximum power to be dissipated by the remote power controller occurs with a zero impedance load, that is, a shorted load or grounded wiring. For a typical 150.+-.20 volt DC system with a 5 amp current limit, the power dissipated in the solid state switch could be as high as 850 watts under normal input voltage conditions. During system voltage transient conditions, the instantaneous peak power dissipation can exceed this level.
Since available power transistors are not capable of withstanding the severe current limiting requirements, circuits have been developed to divert part of the current from the power transistor to power resistors. These helper circuits ensure that during the current switching and limiting operation of the remote power controller, the switching transistor remains in its safe operating area.
One remote power controller with a power sharing circuit portion is disclosed in U.S. Pat. No. 3,697,860, issued to D. E. Baker on Oct. 10, 1972 and assigned to the same assignee as the present invention. That patent discloses a remote power controller having a main switching transistor which is normally driven to a near saturation condition in response to an ON command. The level of load current is sensed by a resistive shunt and compared to a fixed reference value. If the load current tries to exceed the reference, the base drive of the switching transistor is adjusted in a closed loop fashion to maintain constant load current. During current limiting, the voltage across the switching transistor increases as the load impedance decreases. A power sharing circuit is connected in parallel with the main switching transistor to perform part of the required power dissipation, thus permitting the use of smaller and lower cost power transistors. The dissipation resistor can be optimized so that both the main switching transistor and a transistor in the power sharing circuit are operated with approximately the same safety margin with respect to their safe operating area second breakdown regions. The disclosure of U.S. Pat. No. 3,697,860 is hereby incorporated by reference.